1. Field of the Invention
The present invention relates to a power supply device for vehicle and, more particularly, to a power supply device for vehicle using a capacitor in combination.
2. Description of the Prior Art
It is a recent trend that, to be friendly to environments or to improve fuel efficiency, automobiles having an idling-stop function to stop the operation of engine on the occasion of making a stop or an electric power steering for reducing an engine load are commercially available. Further, a hybrid system for positively assisting engine drive is assumed to come into wide use in the future. Furthermore, as to the braking of vehicles, e.g., a regenerative brake system to recover an electric energy from a braking energy is practically utilized.
As described above, although an electric power that automobiles require tends to increase from now on, a problem exits in that a secondary battery cannot sufficiently be charged with an instantaneous electric power in such a manner as in the braking time of vehicles. To meet this, an arrangement of adding in an auxiliary manner a capacitor having a large capacity, typified by an electric double-layer capacitor capable of performing rapid charge and discharge, has been proposed. However, a problem exits in shorter service life of the capacitor in case that the capacitor is left in the state of high voltages close to its rated voltage.
On the other hand, for example, a power supply device for vehicle achieving longer service life of a capacitor was proposed in the Japanese Patent Publication (unexamined) No. 124771/2007.
In this power supply device of the Japanese Patent Publication (unexamined) No. 124771/2007, improvement in service life is achieved by discharging an electric power of a capacitor until its voltage is substantially equal to the voltage of a battery on the occasion when the ignition switch of a vehicle is turned off. In this case, an electric power having been stored in the capacitor is discharged to the battery and the battery is charged.
However, in the power supply device for vehicles incorporating the above-mentioned regenerative brake system, since a regenerative energy is charged by the regenerative brake system at the time of making a stop, the battery is likely to be in the full-charge state as described above.